1. Field of the Invention
The present invention relates to optical transducers and, more particularly, to transducers adapted to convert small displacements into variations of light intensity and thence into corresponding electrical current variations.
2. Description of the Prior Art
Prior art optical transducers, especially of the kind found useful in the past for converting the small deflections associated with sound signals present in a liquid medium into electrical signals, are generally discussed in the R. L. Gravel U.S. patent application Ser. No. 71,893, filed Sept. 4, 1979 for a "Fiber Optic Transducer and Method of Manufacture Therefor", and assigned to Sperry Corporation.
The Gravel fiber optic transducer includes two optical fibers each cut to have end faces substantially perpendicular to an axis and positioned with a small gap between the end faces of the fibers. One fiber is mounted to maintain its end face stationary, for example, while the other is cantilevered to permit displacement of its end face. When the fiber axes lie on a common straight line, light propagating in one fiber will couple with maximum intensity into the other fiber.
An acousto-optic transducer may be constructed in accordance with the Gravel invention by extending an optical fiber between two mounting structures disposed upon a base plate, mounting the fiber under tension to orient the axis along a straight line, bonding the fiber to both mounting structures to maintain the axis orientation thereafter, and cutting the fiber near the end of one mounting structure to obtain two fibers, one mounted to maintain its end face stationary and the other cantilevered to permit transverse displacement of its end face.
It will be clear to those skilled in the art that the displacement sensitivity of the Gravel transducer is governed by the diameter of the core of the optical wave guide. For example, if the free end of the long fiber is caused to vibrate, the light transmitted into and through the associated fixed short fiber varies from one hundred percent when the fiber axes are accurately aligned to zero when one fiber guide is displaced transversely by a distance equal to the core diameter. To obtain maximum pressure sensitivity, the axes of the cooperating fibers may be offset to approximately the fifty percent transmission condition at the prevailing static pressure value.
The Gravel device, when used with a conventional photo-detector-amplifier circuit in a sonar detector, desirably has a sensitivity at least equal to the sea noise ambient and insensitivity to pressure head variation. That is, the Gravel technique is not limited in sensitivity by the phase noise problem that conventionally plagues the interferometric single-mode sensing technique, but is limited only by the fundamentally irreducible electron shot noise characteristics of the photo-detector. Many other optical intensity modulation schemes are undesirably sensitive to pressure head changes.
While the prior art fiber optic approach is preferably selected in certain applications wherein displacement transducers are required, single mode fiber guides are necessary to achieve reasonable sensitivity. Also, the adjustment of the positions of the cooperating fiber ends is relatively critical in three dimensions. Means must be provided for making these adjustments, each of which calls for a tolerance of about one micron; while they may be relatively simple mechanically, there must be precise adjustments in three dimensions for each fiber, invariably adding to the cost of the transducer.